290 EVOLUTION [CHAP. IV 



Letter 211 that if I demonstrate that a considerable amount of sterility 

 would be advantageous to a variety, that is sufficient proof 

 that the slightest variation in that direction would be useful 

 also, and would go on accumulating. 



1. Let there be a species which has varied into two forms, 

 each adapted to existing 1 conditions better than the parent 

 form, which they supplant. 



2. If these two forms, which are supposed to co-exist in 

 the same district, do not intercross, Natural Selection will 

 accumulate favourable variations, till they become sufficiently 

 well adapted to their conditions of life and form two allied 

 species. 



3. But if these two forms freely intercross with each other 

 and produce hybrids which are also quite fertile inter se ) then 

 the formation of the two distinct races or species will be retarded 

 or perhaps entirely prevented ; for the offspring of the crossed 

 unions will be more vigorous owing to the cross, although less 

 adapted to their conditions of life than either of the pure 

 breeds. 2 



4. Now let a partial sterility of some individuals of these 

 two forms arise when they intercross ; and as this would 

 probably be due to some special conditions of life, we may 

 fairly suppose it to arise in some definite portion of the area 

 occupied by the two forms. 



5. The result is that in this area hybrids will not increase 

 so rapidly as before ; and as by the terms of the problem the 

 two pure forms are better suited to the conditions of life than 

 the hybrids, they will tend to supplant the latter altogether 

 whenever the struggle for existence becomes severe. 



6. We may fairly suppose, also, that as soon as any 

 sterility appears under natural conditions, it will be accom- 

 panied by some disinclination to cross-unions ; and this will 

 further diminish the production of hybrids. 



7. In the other part of the area, however, where hybridism 



1 " Existing conditions," means of course new conditions which have 

 now come into existence. And the " two " being both better adapted than 

 the parent form, means that they are better adapted each to a special 

 environment in the same area as one to damp, another to dry places ; 

 one to woods, another to open grounds, etc., etc., as Darwin had already 

 explained. A. R. W. (1899). 



2 After " pure breeds," add " because less specialised." A. R. W. (1899). 



